Field of the Invention
The present invention concerns a method for reducing artifacts during the acquisition of magnetic resonance data from a region undergoing examination in an examination subject by operation of a magnetic resonance device, as well as a non-transitory computer-readable storage medium and a magnetic resonance apparatus for implementing such a method.
Description of the Prior Art
Magnetic resonance imaging is widely known and involves the use of radio-frequency excitation of nuclear spins in an examination subject that deflect the spins from alignment produced by a basic magnetic field (B0 field), in order to be able to measure the signals that result as the spins return to the original orientation. The radio-frequency field for the radio-frequency excitation is generally referred to as the B1 field.
Problems can occur in magnetic resonance scans when metal objects are present, such as implants in patients for example. In spite of the associated complications, the examination of patients who have metal implants has become an important application. The increasing number of patients with orthopedic implants in particular, such as for example, screws, pins, artificial joints, and the like have led to the development of new techniques intended to reduce the considerable image impairments caused by such metal components, because the high contrast for soft tissues offered by magnetic resonance imaging is superior to other methods of examination. At the same time, it also has to be taken into consideration that other types of imaging, such as computed tomography, for example, also exhibit a high degree of metal artifacts.
Magnetic resonance imaging is particularly advisable with post-operative complications because infections, rejection reactions and/or fractures can be diagnosed far more effectively by this imaging modality.
In magnetic resonance imaging, where metal objects are present in the target region, the image artifacts are caused primarily by the distortion of the static basic magnetic field (B0 field) that they produce, which in turn is due to the marked difference in magnetic susceptibility between body tissue and metal. A further known effect is distortions in the dynamic magnetic fields, known as imaging gradients, caused by eddy currents for example. Additionally, disruptions of the radio-frequency field due to, for example, induced radio-frequency currents in the metal object and in the surrounding tissue have recently been the subject of attention.
Known procedures that target the reduction or suppression of artifacts, in particular image distortions and contrast changes due to metal objects, have conventionally been implemented primarily for serious disruptions in the static B0 field in the vicinity of the metal objects. For this purpose, a known technique to use Turbo spin-echo sequences (TSE sequences) with a high band width, for example. Other approaches involve the use of the “view angle tilting” technique (correction of the distortion in the direction of the selection gradient). Another known technique is correction of the distortion in the direction of the slice selection gradient, which is known by the acronyms SEMAC/MAVRIC, as described in the article by B. A. Hargreaves et al., “Metal-Induced Artifacts in MRI”, AJR: 197, 2011, pp. 547-555.
Particularly at high field intensities of 3T or more, instances of shading or signal spikes in the direct vicinity of an implant located in the femur have been observed in hip implants, for example. These artifacts are only visible if the susceptibility effects are slight. A similar phenomenon may also occur in the cases of further rod-shaped metal implants such as gamma nails or interventional catheters or guide wires.
In general such instances of shading or signal spikes arise as a result of B1 inhomogeneities. The externally applied B1 field is in most cases polarized elliptically or circularly and generates an electric field that increases with the radial distance from an isocenter in a body coil. The longitudinal axis of the aforementioned metal foreign bodies is generally aligned in the z-direction and this is generally described in simplified terms as a metal rod. The electric field generates a flow of current through the metal, which current generates a field B1_ind that is superimposed on the B1 field. Due to polarization of the external field and the position of the rod, B1 inhomogeneities of varying intensity occur. In image-acquiring techniques these inhomogeneities can lead to marked signal fluctuations, generally in spin-echo based sequences intended to suppress susceptibility artifacts, and may consequently either simulate or conceal certain medical conditions.
For every object there is a polarization at which the B1 inhomogeneities in the vicinity of the rod are minimized. Yet even in this case, shading can still be detected. Such shading is generally restricted to a local area. Furthermore, an improvement in the homogeneity in the region around the rod is generally linked to a reduction in the homogeneity in the object as a whole.